Segment Information (MIS and MA Revenue by Line of Business) (Details) (USD $) In Millions	3 Months Ended	6 Months Ended
Jun. 30, 2011	Jun. 30, 2010	Jun. 30, 2011	Jun. 30, 2010
Revenue	$ 605.2	$ 477.8	$ 1,182.3	$ 954.4
MA [Member]
Revenue	169.8	151.6	336.9	295.0
Intersegment royalty	2.6	2.4	5.2	4.7
External revenue	167.2	149.2	331.7	290.3
MA [Member] \| Research, Data and Analytics (RD&A) [Member]
External revenue	111.0	105.2	220.6	209.8
MA [Member] \| Risk Management Software (RMS) [Member]
External revenue	40.0	39.2	79.6	72.5
MA [Member] \| Professional Services [Member]
External revenue	16.2	4.8	31.5	8.0
MIS [Member]
Revenue	454.2	344.1	882.8	694.9
Intersegment royalty	16.2	15.5	32.2	30.8
External revenue	438.0	328.6	850.6	664.1
MIS [Member] \| Corporate Finance (CFG) [Member]
External revenue	200.1	127.9	381.9	254.3
MIS [Member] \| Structured Finance (SFG) [Member]
External revenue	86.3	73.1	175.7	144.6
MIS [Member] \| Financial Institutions (FIG) [Member]
External revenue	79.1	63.2	156.0	139.4
MIS [Member] \| Public, Project and Infrastructure Finance (PPIF) [Member]
External revenue	72.5	64.4	137.0	125.8
Eliminations [Member]
Revenue	$ (18.8)	$ (17.9)	$ (37.4)	$ (35.5)

Aggregate revenue recognized during the period (derived from goods sold, services rendered, insurance premiums, or other activities that constitute an entity's earning process). For financial services companies, also includes investment and interest income, and sales and trading gains.

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher SEC
-Name Regulation S-X (SX)
-Number 210
-Article 5
-Section 03
-Paragraph 1

Reference 2: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 225
-SubTopic 10
-Section S99
-Paragraph 2
-Subparagraph (SX 210.5-03.1)
-URI http://asc.fasb.org/extlink&oid=6880815&loc=d3e20235-122688

+ Details

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- Definition

Amount of revenue from external customers for the reportable segment. Such disclosure is presented if the amount is: (a) included in the measure of segment profit or loss reviewed by the chief operating decision maker or (b) otherwise regularly provided to the chief operating decision maker, even if not included in that measure of segment profit or loss.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Statement of Financial Accounting Standard (FAS)
-Number 131
-Paragraph 27
-Subparagraph a
-LegacyDoc This reference is SUPERSEDED by the Accounting Standards Codification effective for interim and annual periods ending after September 15, 2009. This reference is included to help users transition from the previous accounting hierarchy and will be removed from future versions of this taxonomy.

+ Details

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- Definition

Amount of revenue from transactions with other operating segments of the same entity.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Statement of Financial Accounting Standard (FAS)
-Number 131
-Paragraph 27
-Subparagraph b
-LegacyDoc This reference is SUPERSEDED by the Accounting Standards Codification effective for interim and annual periods ending after September 15, 2009. This reference is included to help users transition from the previous accounting hierarchy and will be removed from future versions of this taxonomy.

+ Details

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